With the development of computers having ever increasing computing power, a number of peripheral devices suited for operation in conjunction with the computer have been developed. One of the more useful peripheral devices is known as a graphic recorder or plotter which functions under computer control to provide a graphic output similar to drawings and the like. While the structure of such plotters varies generally with the desired functions and the preferences of plotter manufacturers and users, they all generally comprise a movable pen carriage together with drive means for moving the pen carriage back and forth along a pen carriage path. A media support platen or similar device is positioned beneath the pen carriage path and includes media drive means which engage the media and move it back and forth. In virtually all plotter systems the directions of pen carriage motion and media motion are perpendicular to each other. The desired recording or plot is obtained by computer controlled coordinated movement of the media and pen carriage to record the desired pattern upon the media.
In most instances, the recording element supported upon the pen carriage comprises a recording pen which produces an inked plot upon the media. In their simplest form, recorders have been constructed which support a single pen upon the pen carriage. In more sophisticated plotters, however, a plurality of pens are supported upon the pen carriage. In the latter case, means are provided for selectively engaging and lowering the desired one of the multiple pens into recording contact with the media.
While the use of the presently available recording pens in graphic recorders or plotters provides a substantial benefit to the user, several problems or limitations of recorder performance and capability arise due to the present limitations of the recording pens. Numerous pen designs have been created to meet the needs of graphic recorders. Generally, such recording pens comprise a pen body which includes an ink reservoir, a pen tip and a recording element supported at the end of the pen tip. Means are provided within the pen body and pen tip for communicating the ink from the reservoir to the recording element. Different recording element structures have been utilized to provide different recording characteristics. Two of the more common types are the porous fiber tip recording element and the ball-point pen recording element.
Regardless of the type of pen construction used, several problems or limitations of pen structure have persisted. Most significantly, problems associated with small ink supplies and the wear imposed upon the recording element have limited recording pen life. In addition, the viscosity of the recording ink must be carefully controlled to provide proper ink flow during the ever increasing plotting speeds attained by modern plotters.
It has been found generally that attempts to provide extended pen life together with recording capabilities at higher plotting speeds are not successful with conventional recording pen sturctures such as those described above. Some improvement is realized, however, in the more recently developed pressurized ball-point recording pens. Such pressurized ball-point pens generally comprise an elongated pen body having a sealed ink reservoir which is pressurized with a captive quantity of compressed air or other gas within the ink reservoir which forces the flow of ink to the recording element together with a ball-point recording element in communication with the ink reservoir. While the use of pressurized ball-point pen structures has provided improved recording pen life due to the reduced recording element wear of the ball-point recording element, a number of problems or limitations persist. It has been found, for example, that the ink used in such pressurized pens must have a relatively high viscosity and must include ingredients which provide for the ready formation of a "crust" or layer of dried ink around the recording element ball during nonuse. This crust or dried ink layer is required to prevent the ink from being pushed out through the recording ball element structure by the gas pressure within the ink reservoir. The use of high viscosity inks, in turn, requires substantial gas pressures within the pen and an increase in the writing force exerted by the recorder between the recording pen and the media to cause ink to flow around the ball element during the recording process. In addition, the pressurized ink chamber or reservoir has not heretofore been effectively manufactured using molded plasitc or other materials but has instead been formed of a metal material. The use of metal pen bodies is costly and increases the pen weight. In addition, complications arise due to the corrosive character of many inks which cause them to attack metal pen bodies. Increased pen weight becomes a significant problem in graphic recorders using multiple pens supported upon the pen carriage in that the inertia created by increased pen mass imposes a limitation on plotter speed and performance.
Finally, the shape of the pressurized ink reservoir must provide a relatively small cross-sectional area to ensure that the meniscus formed at the top of the ink supply prevents the pressurizing gas from flowing to the recording end of the pen in the event the pen is placed on its side. In such case, the transfer of the pressurizing gas to the recording end of the pen body raises the possibility of a gas pressure leak which would render the pen useless as well as the possible creation of trapped pressurized gas bubbles within the ink supply when the pen is righted for use in the recorder. If such gas bubbles reach the writing ball, the gas would escape rendering the pen useless.
In view of the foregoing problems, there remains a need in the graphic recording art for a recording pen which provides a long wearing recording element in combination with a large ink reservoir and which permits the use of lower viscosity inks to provide reliable high-speed recording without excessive recording pressure and which may be inexpensively fabricated from a lightweight molded plastic material and which is resistent to damage when placed on its side.